A number of radio systems that are designed to operate in shielded environments, such as mines and long tunnels, are known. One such system is disclosed in U.S. Pat. No. 4,972,505 to Isberg for "Tunnel Distributed Cable Antenna System With Signal Top Coupling Approximately Same Radiated Energy." This system incorporates separate transmitting and receiving antennas and multiple signal taps to adjust the radiated energy.
Another system is described in U.S. Pat. No. 5,230,080 to Fabre et al. for "Ultra-High Frequency Communication Installation." This system is intended for use at a site having multiple underground galleries. The system incorporates multiple repeaters which receive a signal which is retransmitted into the galleries over a combination of radiating and non-radiating cable sections.
A problem with known systems is that the systems require rather larger repeaters or in-line amplifiers, in order to ensure that a radio signal reaches all parts of the underground structure. Frequently, tuned resonant cavity networks have been employed to achieve the desired results, which are both costly and bulky. Additionally, such networks have high signal loss characteristics, which require larger, more powerful, and hence more expensive, amplifiers in order to operate. Such a system is also service intensive, in that frequent service is required in order to maintain the alignment and tuning of the system.